The disposal and/or treatment of waste is as old as civilization itself. The waste products of human activity take many forms, and present numerous disposal and sanitation issues. Of particular importance in the last half century has been the preservation and conservation of our natural resources, including groundwater and land, in particular.
The treatment of wastewater is not a new art, and involves essentially three broad aspects: conveyance, treatment, and disposal. Conveyance is more often associated with large systems, such as municipal sewage treatment systems, and involves transporting the wastewater, with associated solids, to a treatment location.
Various treatment systems are known, including large-scale treatment, common to municipalities, which involve activated sludge processing and/or surface discharge to a river or lake. An onsite wastewater system, however, typically involves a septic tank in combination with a dispersal field. Septic tanks were first introduced in the late 1800's when municipal treatment systems had not yet become an established part of the infrastructure of growing towns and cities. The advent of the Environmental Protection Agency and the population growth in urban areas gave rise to the development of large scale sewer and treatment systems. In recent years, the Environmental Protection Agency has changed its philosophy of placing all homes on large treatment systems with surface discharge, to onsite systems with subsurface dispersal. This has been prompted by increased suburban growth.
Septic systems have not radically changed over the past 100 years. A conventional septic system comprises a septic tank that receives wastewater from a residence, and a dispersal field wherein partially treated wastewater drains into a bed of gravel and soil. The most widely used parameter for estimating the organic pollution of wastewater is the biochemical oxygen demand (BOD). BOD involves the measurement of the dissolved oxygen used by micro-organisms in the biochemical oxidation of organic matter. The higher the number, the greater the organic pollution. Another parameter used in characterizing the degree of pollution is the total suspended solids (TSS). The greater the BOD and TSS, the greater is the formation of a biomat in the drainfield. The biomat, or growth layer of organic matter, accumulates over a period of time and eventually leads to ultimate failure of the drainfield. Thus, measures which reduce BOD and TSS lead to slower growth of the biomat, and thus prolonged life of the septic system. The conventional septic tank generally produces a BOD of about 250 mg/l and a TSS of about 150 mg/l.
As is generally recognized in the art, there are two key factors that reduce the BOD and TSS levels in wastewater: (1) maximizing the oxygen present over a short duration, usually through super-saturation, or (2) increasing the time of treatment. While these principles work well for surface treatment systems, commonly known as activated sludge processes, there has not been a satisfactory and reliable solution for septic systems. There are known septic dispersal systems that incorporate standpipes, or vents, to provide for some natural circulation of air through the dispersal field, but these systems have had little practical effect in reducing the BOD and/or TSS of the waste stream.